The present invention relates generally to photolithography as used in the manufacture of semiconductor devices, and more particularly to controlling the temperature of a reticle in a scanning photolithographic system.
In photolithography as used in the manufacture of electronic or semiconductor devices, a reticle or mask containing a circuit pattern thereon is imaged through projection optics onto a photosensitive or resist covered substrate. Often, very high quality images are required with feature sizes now tending towards 100 nm. In the EUV, extreme ultraviolet, range these sizes may shrink to tens of nanometers. In order to achieve the required high quality images and very small feature sizes, an option for illumination sources for projecting the image of the mask or reticle onto the photosensitive substrate will require the use of electromagnetic radiation having a wavelength in the extreme ultraviolet, EUV. Often, in order to achieve a required exposure dose, the illumination source must be of a relatively high energy or high level of flux. The mask or reticle will absorb a significant fraction of this energy. In the EUV the reticle is reflective. The amount of energy absorbed in the high reflectance zone is thirty-two percent for present coatings for the reflective portion of a mask or reticle and one hundred percent for an absorbing or non reflective portion of a mask or reticle. As a result, the reticle will increase overall in temperature and this increase may be uneven and pattern dependent causing non correctable distortion. Performance is therefore adversely affected. The heating or warming of the reticle is often complicated because the system is generally placed in a vacuum. Additionally, the heating of the reticle is often complicated in a scanning photolithographic system, where a high energy illumination field is scanned across the surface of the reticle. Therefore, there is a need to improve the thermal management of a reticle in a photolithographic system, and in particular a scanning photolithographic system.
The present invention is directed to an apparatus and method for managing the thermal energy absorbed by a reticle. The basic concept is to provide an extra heat/cooling delivery so that the reticle always has a constant heat load and this heat load is independent of the flux on the wafer. The heat load is comprised of an adjustable heater such that the load from this heat source plus the incident flux heat load is maintained constant. A substantially planar cooling element is positioned adjacent the reticle. The cooling element has an aperture therein for transmitting an illumination field from an illumination source. Adjacent the illumination aperture is positioned a heating element. The cooling and heating elements are balanced so that no net temperature change occurs and thus the reticle remains in thermal balance. Heat is provided from two sources, heat from the heating element and heat from the EUV electromagnetic radiation absorbed by the reticle. Selective control of the heating element considering the rate of absorption of the reticle during an exposure dose of EUV electromagnetic radiation results in much reduced flux induced thermal distortion of the reticle, as it is now in an isothermal environment. In another embodiment, the temperature changes of the heating element in a direction perpendicular to a direction of scan may be modified to compensate for an uneven or pattern dependent absorption pattern in the reticle. A suitable inert gas may be introduced between the reticle and the heating and cooling elements to provide cleaning of the reticle by known thermophoretic methods.
Accordingly, it is an object of the present invention to minimize or prevent thermal distortion of a reticle, thereby improving image quality, and minimize any induced overlay error.
It is another object of the present invention to improve thermal management of a reticle in a scanning photolithographic system.
It is an advantage of the present invention that it is easily adjustable for different reticle patterns with different absorption.
It is a further advantage of the present invention that it is readily adaptable to thermophoretic cleaning of the reticle.
It is a key feature of the present invention that heating and cooling elements are simultaneously used.
It is yet another feature of the present invention that a heating element is used and with the energy absorbed by the reticle from the EUV illumination, is maintained at a constant thermal load.
These and other objects, advantages, and features will become readily apparent in view of the following more detailed description.